It is well known to construct motor vehicle body panels, doors, hoods, fenders, tailgates, trunks and deck lids by stamping an outer sheet metal panel and separately stamping an inner sheet metal reinforcing panel and then joining the two panels together by hemming a flange of the periphery of the outer panel over an adjacent edge of the inner panel to secure the panels together. Desirably, the outer panel is slightly larger than the inner panel to provide a border flange portion along the periphery of the outer panel which can be folded over the peripheral edge of the inner panel to define the hem flange which connects the two panels.
When the flange is folded over the peripheral edge of the inner panel with a traditional hemming apparatus and method, the resulting edge of the hemmed panels has a generally smooth, curved or arcuate shape with a diameter of the bend equal to twice the thickness of the outer panel plus the thickness of the inner panel. With inner and outer panels of the same thickness, the diameter of the bend for a standard hemming apparatus and method is equal to three times the thickness of a sheet metal panel. While many standard hemming methods and devices produce a smooth and fair hem or fold line, the relatively large bend diameter of the fold line is believed to reflect light in various directions along the curved fold line which is believed to give the visual appearance that the gap between adjacent hemmed panels of the vehicle body is larger than it actually is, is inconsistent or that the panels are not flush with one another.
In a conventional hemming process, the flange of the outer sheet metal panel is folded flat against the inner panel. In a different hemming process, which may be referred to as an "open hem" process, the flange of the outer sheet metal panel is folded back over the inner panel to secure them together, but the flange remains inclined to the inner panel resulting in a gap between the outer edge of the flange and the inner panel in which foreign material may collect. In use, this gap may also allow the inner panel to shift relative to the outer panel resulting in misalignment of the panels. Thus, this gap between the flange and inner sheet metal panel must be filled with a sealing material to prevent foreign material from collecting in the hem. This sealing operation increases the cost and time to assemble the panels of the vehicle body.